1. Field of the Invention
The present invention relates to a paper transportation device for transporting paper on a paper feed tray to a paper discharge tray. In particular, the present invention relates to a paper transportation device which nips a plurality of sheets of paper picked up from the paper feed tray by a separating roller and a separating pad and separates the papers one sheet at a time.
2. Description of the Related Art
A copier, a facsimile machine, and a scanner or the like includes an image scanning device for scanning an image of an original document, and an image forming device for printing image data scanned from the original document and image data received by facsimile onto printing paper. Many image scanning devices and image forming devices include a paper transportation device, which sequentially transports a plurality of sheets of original documents or printing papers on a paper feed tray, and discharges the original documents or the printing papers onto a paper discharge tray via a prescribed image scanning position and/or an image transfer position. For example, the image scanning device includes a paper transportation device known as an Auto Document Feeder (ADF) which transports an original document from a document tray, where original documents to be scanned are set, onto a paper discharge tray, where scanned original documents are stacked.
In the ADF, as a mechanism for sequentially picking up an uppermost sheet of the original documents stacked on the document tray and feeding the original documents to a transportation path, a paper feeding unit is provided near the document tray. FIG. 4 is a schematic lateral cross-sectional view illustrating a conventional paper feeding unit 70. As illustrated in FIG. 4, the paper feeding unit 70 includes a drive shaft 71, a separating roller 72, a pickup arm 73, a pickup roller 74, a separating pad 75, a separating pad holder 76, a cork pad 77, a catching cover 78, a supporting shaft 79, a swinging bracket 80, a coil spring 81, and a separation pressure adjusting screw 82. The separating roller 72 is rotatably driven by the drive shaft 71. A base end of the pickup arm 73 is supported by the drive shaft 71. The pickup roller 74 is rotatably supported on a leading end portion of the pickup arm 73. The separating pad 75 separates a plurality of sheets of original documents picked up from a document tray (not illustrated) one sheet at a time. The separating pad holder 76 holds the separating pad 75 such that the separating pad 75 can be contacted against and separated from the separating roller 72. The cork pad 77 arranged next to the separating pad holder 76 is mounted removably on the catching cover 78. The swinging bracket 80 is provided below the separating pad holder 76 and the catching cover 78 such that the swinging bracket 80 swings with the supporting shaft 79 as a supporting point. The coil spring 81 is provided between the separating pad holder 76 and one end portion of the swinging bracket 80 on the other side of the supporting shaft 79. The separation pressure adjusting screw 82 is screwed to another end portion of the swinging bracket 80.
In the above-described paper feeding unit 70, the coil spring 81 urges the separating pad holder 76, and the separating pad 75 is contacted against the separating roller 72 under a prescribed separation pressure. Accompanying a start of rotation of the driving shaft 71, the pickup arm 73 swings downward. When the pickup roller 74 at the leading end of the pickup arm 73 rotates while making contact with the original documents, several upper sheets of the original documents among the original documents stacked on the document tray are picked up. The picked-up original documents are fed into a nip portion between the separating roller 72 and the separating pad 75. One sheet of the original documents is separated by a frictional force, and fed into the transportation path 83.
In the above-described paper feeding unit 70, when a separation failure is generated such that original documents are not separated at the nip portion and a plurality of sheets of the original documents are fed into the transportation path 83, or when a pickup failure is generated such that an original document is not fed into the transportation path 83, the separation pressure can be adjusted. That is, the cork pad 77 is removed from the catching cover 78, and a screwdriver or the like is inserted into an exposed through-hole 84 to turn the separation pressure adjusting screw 82. For example, when the separation pressure is insufficient, the separation pressure adjusting screw 82 is screwed inward. An end portion of the swinging bracket 80, which is pressed by the separation pressure adjusting screw 82, at the side of the separation pressure adjusting screw 82 is lowered, and an end portion of the swinging bracket 80 at the coil spring 81 side is elevated. Accordingly, a compressed amount of the coil spring 81 increases so as to increase its urging force. As a result, the separation pressure increases.
FIG. 5 is a schematic lateral cross-sectional view showing another conventional paper feeding unit 90. As illustrated in FIG. 5, the paper feeding unit 90 includes a separating roller 91, a separating pad 92, a separating pad holder 93, a coil spring 94, and a separation pressure adjusting screw 95. The separating pad 92 separates the original documents with the separating roller 91. The separating pad holder 93 holds the separating pad 92. The coil spring 94 urges the separating pad holder 93 to contact the separating pad 92 against the separating roller 91 under a prescribed separation pressure. The separation pressure adjusting screw 95 adjusts the separation pressure by changing a compressed amount of the coil spring 94. An inner guide unit 96 mounted with the separating pad holder 93 is arranged to be capable of swinging around a supporting shaft 97 as a swing center. Accordingly, when the separation failure is generated and it is necessary to adjust the separation pressure, as illustrated in FIG. 5, the inner guide unit 96 is swung upward to expose a bottom surface 96a of the inner guide unit 96. Then, a screwdriver or the like is inserted into a through-hole (not illustrated) formed on the bottom surface 96a to turn the separation pressure adjusting screw 95. Accordingly, a compressed amount of the coil spring 94 changes, and the separation pressure is adjusted.
However, in the paper transportation device including the conventional paper feeding unit 70, a troublesome work is required, i.e., the cork pad 77 is required to be removed from the catching cover 78 for adjusting the separation pressure adjusting screw 82. Since a center axis of the separation pressure adjusting screw 82 does not coincide with a center axis of the coil spring 81, the separation pressure cannot be adjusted under a stable state. In the paper transportation device including the conventional paper feeding unit 90, a structure for swinging the inner guide unit 96 is required. Therefore, the complexity of the mechanism of the paper transportation device is increased, causing an increase in costs.